ACtion Magazine - March 2014 - (Page 26)

By Pete Meier A ll cars and light trucks today rely on a computer to control the A/C compressor. And the computer in charge is usually the Powertrain Control Module (PCM). That's because the A/C compressor has an impact on fuel efﬁciency and emissions, and that is the realm of the PCM. Other computers can be involved in the decision, as well as controlling other HVAC functions like blower speed and mode door positioning. But the PCM has the ﬁnal say when it comes to the operation of the compressor. Control unit logic Diagnosing computer controlled circuits is not hard if you consider how it all works. A computer can only function according to its programming. Before it can make any sort of decision or take any kind of action, it needs information. That information is supplied by input devices, like a system pressure sensor or an evaporator temperature sensor. The information collected is plugged into the programming, and the resulting decision is carried out through an output device, like the compressor clutch coil relay. The control module also needs to know that the decision was carried out successfully, and it determines this through 26 ACTION * March 2014 feedback. Feedback can be a dedicated sensor or switch located in the actuated component itself, or it can be provided by an existing input device that the control module monitors for change. An example of the latter is the change in A/C system pressure seen by the PCM when the compressor engages. Now, the control module has a relatively simple mind. It can only understand ones and zeros, yes/no, high/low or on/off. The information it receives has to be in this digital format. If an input signal doesn't reach its full minimum or maximum level, the computer won't understand the message. Some input signals are varying voltages, like those from a potentiometer, but the computer doesn't use them until they pass through an analog/digital converter (A/D converter) internal to the control module. Output signals work the same way. The control module can turn a circuit on by supplying power, or by completing the path to ground through an internal electronic switch. Power is supplied by a device called a high side driver, and a ground is supplied by a low side driver. The circuit can be on or off, or it can be cycled on/off many times per second to produce a varying voltage output. Cycling the circuit on and off is often referred to as pulse width modulation (PWM), but is more accurately termed duty cycle.

Table of Contents for the Digital Edition of ACtion Magazine - March 2014

Coolant pump replacement
Computer controls for Air conditioning replacement
Outlook
Freeze Frame
Leonard's Law
Virtual View
Service Port
Last Watch
By the Numbers
Cooling Corner
Industry News
Association News
New Products

ACtion Magazine - March 2014